The goal of the proposed research is to identify the mechanism of action of thyroid hormones on mitochondria. There is considerable evidence, including the description of specific mitochondrial receptors for thyroid hormones, that thyroid hormones have a direct effect on mitochondria. Thyroid hormones increase mitochondrial protein synthesis in vitro and in vivo as well as increase mitochondrial respiration, intramitochondrial ATP concentrations and mitochondrial volume (so-called "swelling"). The proposed studies investigate the mechanism by which thyroid hormones produce these effects on mitochondria, the function of the protein synthesized by mitochondria in vitro and in vivo and the presence of specific mitochondrial receptors for thyroid hormones.